Flexible liquid desiccant heat and mass transfer panels with a hydrophilic layer

ABSTRACT

Provided are flexible panel devices that use desiccants for heat and mass transfer processes, including but not limited to air conditioning systems, for example, liquid desiccant air conditioning (LDAC) applications wherein the liquid desiccant is contained in a panel that comprises at least one hydrophilic separation layer, which allows water vapor transfer between the air and liquid desiccant and enable dehumidification and humidification of the air. The flexible panel devices can be installed on an absorber (conditioner) side or a desorber (regenerator) side or both of a LDAC system. The devices have two flexible layers, at least one of which comprises a flexible and water vapor permeable hydrophilic separation layer, that form a desiccant flow channel and a desiccant flow distributor located therein. The two flexible layers may both be permeable hydropholic separation layers, or they may comprise one permeable hydrophilic separation layer along with another layer that may be a non-porous structure or a water-vapor permeable hydrophobic separation layer.

TECHNICAL FIELD

This disclosure relates to flexible panel devices that use desiccantsfor heat and mass transfer processes, including but not limited to airconditioning systems. Specifically, devices disclosed herein areparticularly useful in liquid desiccant air conditioning (LDAC)applications wherein heat and mass transfer is achieved with a panelthat comprises one or more hydrophilic separation layers that arewettable by the desiccant. A desiccant flow distributor located in thepanel is hydrophilic and is fabricated to provide excellent wicking anddrawing of the desiccant through the panel.

BACKGROUND

The use of liquid desiccants for dehumidification of air has been knownfor well over 75 years. The application of liquid desiccants indehumidification applied in heating, ventilating, and air conditioning(HVAC) systems has been worked on for many years. Open absorptionsystems for air conditioning are desirable due to their relativelysimple design and driving energy at relatively low temperatures. Liquiddesiccant air conditioning (LDAC) is an exemplary open absorptionsystem.

Membrane modules have been researched and attempted for use in LDACsystems. Some module designs incorporated three fluid paths: one fordesiccant, one for air, and one for coolant; and other designsincorporate two fluid paths: one for desiccant and one for air. Certaindesigns have provided benefits on the performance of the absorber sideof the system but not on the desorber side, and overall commercialsuccess of liquid desiccant air conditioning (LDAC) systems has beenextremely limited.

SUMMARY

Provided are heat and mass transfer panels, heat and mass transfermodules, and methods of making and using the same.

In a first aspect, a heat and mass transfer panel for water vaporexchange with a liquid desiccant is provided, the panel comprising: adesiccant flow channel defined by a first flexible porous layer and asecond flexible layer, at least one of which comprises a flexiblehydrophilic separation layer; a desiccant inlet and a desiccant outletto the desiccant flow channel; and a flexible desiccant flow distributorlocated in the desiccant flow channel.

Another aspect provides heat and mass transfer modules comprising: oneor more panels disclosed herein assembled among one or more air channellayers or air gaps; and an air inlet and an air outlet.

Other features that may be used individually or in combination withrespect to any aspect of the invention are as follows.

Both the first and the second flexible layers may comprise a flexiblehydrophilic water-vapor permeable separation layer. Or, the firstflexible layer may comprise a flexible hydrophilic water-vapor permeableseparation layer and the second flexible layer may be a non-porous layeror a hydrophobic water-vapor permeable separation layer. The flexiblehydrophilic water-vapor permeable separation layer or layers mayindependently comprise a membrane, a woven mesh, a nanofiber media, anelectrospun fiber media, a glass fiber media, a nonwoven melt blownfiber media, a corrosion-resistant metal, a ceramic media, orcombinations thereof. The flexible hydrophilic water-vapor permeableseparation layer or layers may independently comprise a micro-filtrationor an ultra-filtration membrane comprising a hydrophilic nylon (PA)membrane, a hydrophilized polyethersulfone (PES) membrane, ahydrophilized polysulfone (PS) membrane, a hydrophilized polyvinylidenefluoride (PVDF) membrane, a hydrophilic polyacrylonitrile (PAN)membrane, a hydrophilized polypropylene (PP) membrane, a hydrophilizedpolyethylene (PE) membrane, a hydrophilized polytetrafluorethylene(PTFE) membrane, a hydrophilized polycarbonate (PC) membrane, ahydrophilized ethylene chlorotrifluoroethylene (ECTFE) membrane, orcombinations thereof.

In some embodiments, the desiccant flow distributor is effective touniformly spread desiccant under head pressure conditions of 12 inches(30.5 cm) of water or less. The desiccant flow distributor may beeffective to uniformly spread desiccant under head pressure conditionsin the range of atmospheric pressure to less than or equal to 12 inches(30.5 cm) of water.

The desiccant flow distributor may comprise a hydrophilic material thatcomprises a polymeric material, a natural fiber, or combinationsthereof. The desiccant flow distributor may comprise a hydrophilicpolymer material that comprises a membrane, an open cell foam, a porousnonwoven material, a porous woven material, or combinations thereof. Thedesiccant flow distributor may comprise a hydrophilic polymer materialthat comprises a rail film, an extruded web material, an aperturedpolymeric film, or combinations thereof. The desiccant flow distributormay comprise a hydrophilic natural fiber that comprises cellulose. Thedesiccant flow distributor may comprise an open cell foam of ahydrophilized polyether urethane or a hydrophilized polyester urethane.The desiccant flow distributor may comprise one or more draw-and-dripfeatures at an outlet end of the distributor; wherein the draw-and-dripfeatures are effective for facilitating uniform flow through the panel.

The non-porous layer may comprise polyethylene, cast, polypropylene,oriented polypropylene, PET (polyethylene terephthalate), bi-axiallyoriented PET, bi-axially oriented PET with aluminum or gold vapordeposited on the surface, PA (polyamide), PVC (polyvinylchloride), EVOH(ethylene vinyl alcohol) and/or co-extruded/multilayer filmconstructions thereof.

The heat and mass transfer panels may further comprise an air channellayer. The heat and mass transfer panels may further comprise adesiccant distribution header.

In one or more embodiments, the heat and mass transfer panels, uponcontact with air having a water vapor pressure higher than theequilibrium vapor pressure of the desiccant, are effective to transferwater vapor from the air to a desiccant flowing through the desiccantchannel. In one or more embodiments, the heat and mass transfer panels,upon contact with air having a water vapor pressure lower than theequilibrium vapor pressure of the desiccant, are effective to transferwater vapor from the desiccant to the air.

The heat and mass transfer modules may further comprise two end platesbetween which the one or more panels and the one or more air channellayers are assembled. The end plates may be mechanically fastenedtogether. In some instances, there are fewer desiccant inlets thandesiccant outlets. In other instances, there are fewer desiccant outletsthan desiccant inlets.

Further aspects provide methods for water vapor exchange between air anda liquid desiccant, the methods comprising: contacting any paneldisclosed herein with air having a water vapor pressure different fromthe equilibrium vapor pressure in a desiccant flowing through thedesiccant flow channel; wherein the humidity of the air after contactwith the panel is different from the humidity before contact with thepanel.

When the water vapor pressure of the air is higher than the equilibriumvapor pressure of the desiccant, the method may further comprisetransferring the water vapor from the air to the desiccant, and thehumidity of the air after contact with the panel is less than thehumidity before contact with the panel. When the equilibrium vaporpressure of the desiccant is higher than the water vapor pressure of theair, the method may further comprise transferring the water vapor fromthe desiccant to the air, and the humidity of the air after contact withthe panel is more than the humidity before contact with the panel. Thedesiccant flow distributor may comprises one or more draw-and-dripfeatures at an outlet end of the distributor; wherein the draw-and-dripfeatures are effective for facilitating uniform flow through the panel.

Another method is a method of making a heat and mass transfer panel, themethod comprising: forming a desiccant flow channel defined by a firstflexible layer and a second flexible layer, at least one of whichcomprises a flexible hydrophilic water vapor-permeable separation layer;locating a flexible desiccant flow distributor in the desiccant flowchannel; assembling the first flexible layer, the second flexible layer,and the flexible desiccant flow distributor; and providing or forming adesiccant inlet and a desiccant outlet to the desiccant flow channel.

A further aspect is a desiccant flow distributor comprising: ahydrophilic structure comprising a polymeric material, a natural fiber,or combinations thereof; and one or more draw-and-drip features at anoutlet end of the structure; wherein the draw-and-drip features areeffective for facilitating uniform flow therethrough. The desiccant flowdistributor may comprise a hydrophilic polymeric material that comprisesa membrane, an open cell foam, a porous nonwoven material, a porouswoven material, or combinations thereof. The one or more draw-and-dripfeatures may comprise a series of edges such that the linear edge nearor at the outlet end of the structure is longer than the linear edge atthe inlet end of the structure. The one or more draw-and-drip featuresmay comprise a series of shapes defined by or at the edges of the outletend.

These and other aspects of the invention are described in the detaileddescription below. In no event should the above summary be construed asa limitation on the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention described herein and are incorporated inand constitute a part of this specification. The drawings illustrateexemplary embodiments. Certain features may be better understood byreference to the following detailed description when considered inconnection with the accompanying drawings, in which like referencenumerals designate like parts throughout the figures thereof, andwherein:

FIG. 1 is a schematic illustrating condensation followed by hydraulicbreakthrough resulting from prior art structures;

FIG. 2 is a process flow chart for an exemplary flexible panelfabrication system;

FIGS. 3A-3B show an exemplary panel in an expanded schematic view (FIG.3A) and in an assembled schematic view (FIG. 3B);

FIG. 4 is a schematic view of a combination of an exemplary air channellayer and air channel seals;

FIGS. 5A-5B show an exemplary module in an expanded schematic view (FIG.5A) and in an assembled schematic view (FIG. 5B);

FIG. 6 is an embodiment of a flexible liquid desiccant heat and masstransfer panel with two air channel layers in a module assembly havingtwo end plates; and

FIGS. 7A-7D provide schematic depictions of exemplary draw-and-dripfeatures having various shapes.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. It will be understood, however, thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

Provided are improved flexible panel devices that use desiccants forheat and mass transfer processes, including but not limited to airconditioning systems, for example, liquid desiccant air conditioning(LDAC) applications allowing heat and water vapor transfer between theair and liquid desiccant, which enable dehumidification and/orhumidification of the air. The flexible panel devices may be installedon an absorber (conditioner) side or a desorber (regenerator) side orboth of a LDAC system.

The devices have two flexible layers, at least one of which comprises aflexible and hydrophilic separation layer, that form a desiccant flowchannel and a desiccant flow distributor located therein. The twoflexible layers may both be hydrophilic separation layers, or they maycomprise one hydrophilic separation layer along with another layer thatmay be a non-porous structure or a water-vapor permeable hydrophobicseparation layer. An air channel layer is an optional layer.

The at least one flexible porous hydrophilic separation layer controls athin film of liquid desiccant within the pores and on the surfaces ofthe separation layer via surface tension and capillary action. Theseparation layer(s) are in contact with a hydrophilic desiccant flowdistributor which provides hydrodynamic control of a falling column ofliquid desiccant between the separation layers. The use of poroushydrophilic separation layer(s) in flexible liquid desiccant heat andmass transfer panels has significant advantages over the use of poroushydrophobic separation layers.

The following terms shall have, for the purposes of this application,the respective meanings set forth below.

A “panel” is a fundamental structure for achieving mass and/or heattransfer. Panels may provide multiple functionalities such as watervapor separation, distribution of a desiccant, and management ofcondensation. Panels may comprise two layers, at least one hydrophiliclayer and another layer, to form a channel through which desiccantflows. The hydrophilic layer facilitates water vapor transfer. Thechannel may contain a desiccant flow distributor to facilitatesubstantially uniform flow through the channel.

A “module” is an assembly of several panels to achieve mass and/or heattransfer in practical commercial quantities.

“Flexible layers” and “flexible panels” refer to structures that arenon-rigid and can be rolled onto itself and unrolled without damage. Inone or more embodiments, such layers or panels may be rolled 180 degreesaround a radius that is less than or equal to five (or two and one-half,or even less than or equal to one) times the thickness of the layerwithout damage.

“Hydrophilic” means that the liquid desiccant is able to invade and wetthe pores of the separation layer. This effect can be quantified by Eq.(1) which gives the criterion for a liquid to invade a textured soliddepending on its intrinsic wettability with the solid and the details ofthe textures:

$\begin{matrix}{{\cos \; \theta} \geq \frac{1 - \varphi_{s}}{r - \varphi_{s}}} & (1)\end{matrix}$

where θ is the contact angle of the liquid with the solid without anytextures (i.e. smooth), φ_(s) is the solid fraction (between 0 and 1),and r (≧1) is the ratio of true surface area of the solid to itsprojected area. For a porous solid, r is infinity, which implies thatθ≦90°, i.e. any liquid with contact angle less than 90° will eventuallyinvade a porous material. Eq. (1) merely predicts whether or not a givenliquid will invade a porous solid, but it does not predict how quicklythis will happen. This feature of a porous material distinguishes itfrom solids having textures only on their surfaces where the conditionfor liquid invasion is more restrictive. For example, for φ_(s)=0.1 andr=2, θ≦62°, implying that only liquids with contact angle less than 62°will be able to invade the solid textures. Thus “hydrophilic” in thiscontext means θ≦90°, preferably θ should be as small as possible andpreferably θ≦75°.

A “hydrophilic separation layer,” therefore, refers to a structure thatis wettable by the liquid desiccant solutions. Exemplary such structuresinclude but are not limited to: a membrane, a woven mesh, a nanofibermedia, an electrospun fiber media, a glass fiber media, a nonwoven meltblown fiber media, a corrosion-resistant metal, a ceramic media, orcombinations thereof, which are hydrophilic by virtue of the materialsused to fabricate the layer and/or by treatment. Exemplary membranes aremicro-filtration or ultra-filtration membranes.

In addition, “hydrophobic water vapor-permeable separation layer” and“hydrophobic separation layer” refer to a structure that is porous towater vapor but is not wettable by the liquid desiccant solutions.

A “liquid desiccant” is a hygroscopic material which has the ability toboth absorb or desorb water vapor into or from solution based on partialpressure differences. Examples of suitable desiccants are halide salts(such as lithium chloride, calcium chloride, and mixtures thereof, andlithium bromide) and glycols (such as triethylene and propylene glycol).

A “porous separation layer” is the layer of material in the flexiblepanel that controls the interface between the liquid desiccant and theair (or any gas) to be dehumidified or humidified. “Control” means topromote both heat and mass transfer between the liquid desiccant and theair while ensuring that liquid desiccant is not released, aerosolized,and entrained in the air stream. Liquid desiccants are typicallycorrosive salts (i.e. lithium chloride, calcium chloride) and can causemany problems in heating, ventilation, and air conditioning (HVAC)systems if they are allowed into the air stream. Corrosion of equipmentcomponents, corrosion of ductwork, and health and safety concerns haveprevented the widespread use of liquid desiccants in HVAC systems andimproved solutions to liquid desiccant control are needed.

It has been found that the use of hydrophobic water vapor permeableseparation layers to control the liquid desiccant and air interface asdescribed in the prior art have some limitations. When used in LDACsystems in instances where the desiccant enters the heat and massexchanger at a temperature lower than the dew point, one limitation isthat condensation can occur on the air side surface of the separationlayers as well as within the pores of the layer. Beads of liquiddischarging to the air side will bead on the outer surface of thehydrophobic membrane as the bulk of the outer surface is stillhydrophobic and not readily wetted. The formation of beads will causeair flow to be blocked on the air side and beads have a much higherchance of being aerosolized or otherwise carried down the air stream.This phenomenon will continue to spread as condensation continues toform bridges across the layer. The ability of the hydrophobic watervapor permeable separation layer to adequately control the desiccant canonly be restored if the layer is completely dried to eliminate allliquid bridging. Even if the hydrophobic separation layer is dried,there may be residual salt left on the internal surfaces of the layerwhich could promote rewetting.

In addition, capillary condensation can occur at temperatures above thedew point (below the saturation vapor pressure) within the pores of thelayer. The probability of occurrence increases if the pore structure ofthe separation layer is small as can be found in the pore distributionof micro-filtration membranes, and in ultra-filtration, nano-filtration,and osmotic membranes. Any condensation that fills the pores and bridgesthe thickness of the layer will create a hydraulic path where desiccantleakage can occur. Turning to FIG. 1, which is a schematic illustratingcondensation 14 followed by hydraulic breakthrough 18 resulting fromprior art structures, if the pressure on the desiccant side 12 of ahydrophobic separation layer or membrane 10 (P_(desiccant)) is greaterthan the pressure on the air side 16 (P_(air)), hydraulic leaks 18 willoccur and desiccant will be discharged to the air stream side. This canbe most problematic on modular and desiccant flow circuit designs wherethe desiccant pressure is ≧0.5 psig (14 inches or 35.5 cm) or on designswhere there is a totally open desiccant channel with no desiccant flowdistributor.

Furthermore, any surface contamination due to materials such assurfactants, greases, or oils can affect the surface energy of thehydrophobic water vapor permeable separation layers further complicatingthe ability of the layer to effectively retain the desiccant. This canalso be most problematic on modular and desiccant flow circuit designswhere the desiccant pressure is ≧0.5 psig (14 inches or 35.5 cm) or ondesigns where there is a totally open desiccant channel with nodesiccant flow distributor.

A limitation when using a hydrophobic water vapor permeable separationlayer is that the air/desiccant interface occurs on the desiccant sideof the layer. This means that there is a heat transfer resistance in theheat and mass exchanger due to the stagnant layer of air occurringwithin the pore structure of the layer. By moving the air/desiccantinterface to the outside of the hydrophilic layer(s), high heat transferof water through the hydrophilic layer(s) is achieved.

The heat transfer resistance, R_(th,sep) posed by the hydrophobicseparation layer can be determined as: R_(th,sep)=t/k where t and k arethe thickness and thermal conductivity (determined by theporosity-weighted sum of the thermal conductivities of the layermaterial and air) of the separation layer. For instance, for apolypropylene separation layer of 50 μm thickness and 70% porosity,k≈0.7k_(air)+0.3k_(PP)=0.0796 W/mK and R_(th,sep)=6.281×10⁻⁴ m²K/W. Bycontrast, for a hydrophilic separation layer of the same thickness andsimilar thermal conductivity, k≈0.7k_(water)+0.3k_(PP)=0.48 W/mK andR_(th,sep)=1.042×10⁻⁴ m²K/W, which is over six times smaller than thehydrophobic case and, as a result, will yield higher heat transfer ratesfor a given surface area and temperature difference.

The mass transfer resistance R_(m,sep) posed by the hydrophilicseparation layer may be higher or lower than the hydrophobic casedepending on whether the liquid desiccant within the layer is stationaryor mobile. The mobility of the desiccant is difficult to determinetheoretically and can be manipulated by suitable choices of separationlayer and desiccant distributor materials. In addition, the magnitude ofR_(m,sep) has to be compared with the other two mass transferresistances (air-side and desiccant-side) in the system in order toestablish whether R_(m,sep) could be a significant bottleneck for masstransfer. There is an advantage in using a hydrophilic separation layerin that it stays in intimate contact with the desiccant distributor dueto surface tension and enhances overall heat and mass transfer throughthis contact. These considerations make experimental testing necessaryin order to determine mass transfer performance of the hydrophilicseparation layers.

The Panels

In this invention, the use of a porous hydrophilic separation layer haseliminated the stagnant air layer present in the hydrophobic water vaporpermeable separation layer as described in the prior art. A particularlyuseful material for use as a desiccant flow distributor is a poroushydrophilic open cell foam. The effective use of the porous hydrophilicseparation layer is enabled by the use of a gravity feed desiccant flowsystem in combination with a porous hydrophilic open cell foam as thedesiccant flow distributor. The structure and shape of the foam can bemanipulated to optimize the desiccant flow between two poroushydrophilic separation layers or between one hydrophilic separationlayer and another layer that may be a non-porous structure or awater-vapor permeable hydrophobic separation layer. When liquid is fedinto the top of a panel with this construction, the open cell foamspreads the liquid uniformly between the separation layers as it flowsdownward due to the effect of gravity. The porous hydrophilic separationlayer, unlike a hydrophobic material, wets out with the liquid and allthe pores become filled. The stagnant air layer is unable to form withinthe separation layer. Furthermore, surface tension forces keep thehydrophilic separation layer and desiccant distributor in intimatecontact and prevent any air entrapment in between. It is important thatthe open cell foam desiccant distributor be hydrophilic upon initialwetting with the desiccant. Once it has been wetted, it should remainwet as long as the vapor pressure gradient of the environment in whichthe panel resides favors water absorption. After initial wetting, evenif the desiccant loaded panels are dried with high temperature dry air,the panels should re-wet without problem due to the salt left behindfrom the evaporation of the water from the drying operation. In otherwords, the porous hydrophilic separation layer should be hydrophilicenough to initially thoroughly wet and fill out all the pores under thegravity flow condition.

The effect of the fluid flowing through the open cell foam desiccantdistributor to a discharge at the base of the panel is to create acontrolled desiccant flow with minimal positive pressure. By positivepressure, it is meant that the liquid pressure between the separationlayers is only slightly above atmospheric pressure and does not exceedthe height of the liquid column provided in the desiccant distributionheader. The combination of the minimal positive pressure gradient with ahydrophilic porous separation layer which has a fairly high flowresistance is to create a panel where the desiccant flow is sequesteredwithin the pores of the separation layer in an extremely thin, highlyspread liquid layer on the air side of the separation layer. This can bereferred to as a stable film in that under the airflows per unit ofactive heat and mass exchanger surface area anticipated in a LDAC, it isvirtually impossible to aerosolize and entrain desiccant in theairstream. In addition, the wetted separation layers stay in intimatecontact with the desiccant distributor due to surface tension of theliquid. This eliminates any air gaps and maximizes heat and masstransfer efficiency.

Also, when flow into the top of the panel is stopped, for example, whenused in an LDAC and the unit is turned off, the desiccant can bestabilized within the pore structure of the hydrophilic separation layerdue to capillary forces alone. In some embodiments, desiccant may drainout, but will uniformly distribute again upon introduction of desiccantflow. In any case, the desiccant is prevented from forming drips orleaks which could enter the air stream and be entrained into the air inthe form of an aerosol.

The arrangement of a porous hydrophilic separation layer such as amembrane in contact with a hydrophilic porous material such as open-cellfoam effectively solves the widespread problem of carryover encounteredin liquid desiccant systems. This solution originates primarily from thefact that the liquid is held tightly on the surface via surface tension,and within the pores of the separation layer by virtue of capillaryforces, which are far greater than the shear imposed by the passing airstream. This can be shown as follows.

Ishii and Grolmes (M. Ishii & M. Grolmes (1975), Inception criteria fordroplet entrainment in two phase concurrent film flow, AICHE Journal,Vol 21 2 308-318) showed that a gas stream passing over a liquid streamcan dislodge droplets from the liquid if the shear imposed by the gasovercomes the surface tension forces, resulting into the followingcriterion for droplet entrainment:

$\begin{matrix}{{\frac{\mu_{f}v_{g}}{\gamma}\sqrt{\frac{\rho_{g}}{\rho_{f}}}} \geq {11.78N_{\mu}^{0.8}{Re}_{f}^{{- 1}/3}\mspace{14mu} {for}\mspace{14mu} N_{\mu}} \leq \frac{1}{15}} & (2)\end{matrix}$

where μ_(f), ρ_(f), and γ are the viscosity, density, and surfacetension of the liquid, respectively, and ρ_(g) and ν_(g) are the densityand velocity of the gas stream.

$N_{\mu} = \frac{\mu_{f}}{\lbrack {\rho_{f}\gamma \sqrt{\frac{\gamma}{g\; {\Delta\rho}}}} \rbrack^{1/2}}$

is referred as viscosity number and the liquid Reynolds number

${{Re}_{f} = \frac{4\rho_{f}v_{f}\delta}{\mu_{f}}},$

where Δρ is the density difference between the liquid and gas, and δ isthe liquid film thickness. For air-aqueous LiCl system, N_(μ)=0.0097which is <1/15, indicating that the criterion of Eq. (2) is applicable.If the liquid completely wets (i.e. contact angle, θ=0 deg.) theseparation layer, de Gennes et al. (P. G. de Gennes, F. Brochard-Wyart,& D. Quere, Capillary and Wetting Phenomena, Springer Publishing 2004)have shown that its thickness is governed by the balance of surfacetension and van der Waals forces and is typically on the order of 1 nm.If θ>0 deg., microscopic features of the separation layer will remainemerged (i.e. dry) depending on where the local contact angle equals theequilibrium value. We can consider the case of θ=0 deg. with δ=1 nm. Aconservative estimate of ν_(f) would be setting it equal to the bulkfluid velocity in the porous material (such as open-cell foam), whichwould be given by Darcy's law to be about 0.0075 m/s under typicalconditions described previously. This gives Re_(f)=7.2×10⁻⁶. ν_(g) canbe estimated by the typical air CFMs encountered in liquid desiccantsystems, giving ν_(g)=0.8 m/s. Substituting all these values in thecriterion of Eq. (2), the left hand side turns out to be 1.327×10⁻³,whereas the right hand side is about 14.5, which is five orders ofmagnitude greater than the left hand side value. This suggests thatdesiccant droplet entrainment (i.e. carryover) is highly unlikely underthe typical air flow conditions encountered in liquid desiccant systems.In fact, even if δ=1 mm (for instance, for a few seconds due tocondensation of water vapor on separation layer surface), the left handside was calculated to be two orders of magnitude smaller than the righthand side, indicating that droplet entrainment is highly unlikely.

Flow resistance of the open cell foam desiccant distributor is also animportant consideration in the design of the panel. Flow through such amedia is governed by Darcy's law. See A. E. Scheidegger, The physics offlows through porous media, Third ed., University of Toronto Press,Toronto (1974) and K. Boomsma, D. Poulikakos, The effects of compressionand pore size variations on the liquid flow characteristics in metalfoams, J. Fluids Eng. (2002) 124, pp. 263-272. Darcy's law states thatthe pressure drop, ΔP across the media is proportional to media lengthL, fluid viscosity μ, and fluid velocity ν, and inversely proportionalto media permeability, K:

$\begin{matrix}{\frac{\Delta \; P}{L} = {\frac{\mu}{K}v}} & (3)\end{matrix}$

Darcy's law is applicable for slow moving flows characterized byReynolds number Re=ρ√{square root over (K)}ν/μ<O(1), where ρ is thefluid density. Experiments were carried out with hydrophilic open-cellfoams available from UFP Technologies (Type HS) to study water flowbehavior. The key parameter varied was media length, L (95 mm and 160mm); ΔP was kept constant at 500 Pa (i.e. 2″ of water column). Foamwidth w and thickness t were 32 mm and 6.35 mm, respectively. Tests wereconducted with water at room temperature and the resulting flow rate Ωwas measured from which ν=Ω/(wt) was calculated. Foam permeability K wascalculated to be 3.2×10⁻⁹ m² from Eq. (3) using one of the flow ratemeasurements. The calculated value was validated by the good agreementfound between predicted and measured flow rates at other media lengths.To confirm the applicability of Darcy's law, Re was calculated and foundto be 0.6.

Equation (3) and the Reynold's number can be utilized to determine thebulk flow capacity of a specific panel design and used for optimizationof the heat and mass exchanger.

Assembly of Panels

It is important that the separation layers be attached or affixed to thedesiccant distributor in some manner to make an integral panel. Sideseams may be used or created using tapes, adhesives, ultrasonic welding,thermal welding or any other method of attachment. The seams do not haveto be liquid tight seals as the low pressure of operation and thecapillary action of the materials will control the desiccant within andon the panel. An exemplary tape is a closed cell foam acrylic tape, forexample, one of 3M VHB™ tapes identified as #4955 and 4959F would beuseful.

Side seams may be eliminated altogether and the panel will stillfunction provided that head pressure is very low, in the range of <1″(2.5 cm) of water column. An example construction would be to attach oraffix the separation layers to the desiccant distributor by using alight adhesive coating between the layers and laminating the assemblytogether. Additional methods of laminating the separation layers to thedesiccant distributor include but are not limited to heat lamination,open flame lamination, ultrasonic point bonding, and adhesive pointbonding.

Should ultrasonic welding be used, some typical parameters include:

Branson Ultrasonic Welder using a welding horn having an approximatesize of 10 inches (25.4 cm) by 0.25 inches (6.3 mm) with the followingsettings.

Weld Pressure: 40-60 psi

Weld Time—0.5 sec to 1.5 sec

Weld Hold Time—0.5 sec

Trigger Force—set at 12

Down speed—set at 30

Amplitude—set at 100%.

For commercial purposes, it is desirable to arrange the layers in anefficient and orderly manner. One exemplary process for making aflexible LDAC separation panel is as follows: obtain the materials forthe various layers in rolled or bulk form; unwind and/or feed the layersin a stacked form, attach or affix the layers together; cut the layersto length; and affix at least a desiccant inlet and optionally adesiccant outlet. FIG. 2 provides process flow chart for an exemplaryflexible panel fabrication system prior to attachment of fluidconnections which incorporates an open cell foam slab feeder. In FIG. 2,it is shown that two porous hydrophilic separation layers 101, 103 areunwound 105 and a hydrophilic open cell foam 107 is provided by a slabfeeder 109 between the two separation layers. Feed rollers 111 and guiderolls 113 form a structure for receipt by a seamer 115 that providesside seams by any preferred method such as rotary ultrasonic welding,thermal welding, tape or adhesive application. A cutter 117 cuts theseamed structures to size to form heat and mass transfer panels, and thepanels are piled in a stack 119.

FIG. 3A shows an expanded schematic view and FIG. 3B shows an assembledschematic view of an exemplary panel 100 comprising two flexiblehydrophilic separation layers 102, a desiccant flow distributor 104, andadhesive tape 106.

The materials, in particular the open cell foams, used in this inventionare also resilient and a small amount of pressure on the ends of stackedpanels can be applied to insure conformance of panels relative to eachother. The use of open cell foams, for example, allows for a highlyresilient panel and assembled module design which can tolerate both thehydraulic and thermal expansion required in the application. It isexpected that assembled modules incorporating the flexible panels willneed to withstand conditions below freezing as well as temperatures upto 140° F. in operation. This type of flexible construction allowsmaterials to move relative to each other as the panel or moduleexperiences changes in temperature and pressure. This minimizes stressconcentrations throughout the assembly and prevents damage to thestructure. The panels are highly durable and can be folded, dropped,compressed, and impacted without affecting functionality.

It would also be useful in the design of a panel to make any seam usedon the leading (and/or trailing) edge of the panel conform to a roundedshape to minimize any turbulence and drag in the air channel. This willminimize parasitic losses.

Assembly of Modules

In general terms, assembly of a module involves placing one or aplurality of panels and optionally air flow layers/plates in astandalone unit. The panel or plurality of panels may be containedwithin a frame, such as two plates, that allows for desiccant inflow andoutflow through the desiccant channel as well as air flow along theouter surface of the panels as facilitated by an air flow layer orplate, which may be affixed to the panel or which may be provided byassembling panels such that there are air gaps between them. FIG. 4provides a schematic view of a combination of an exemplary air channellayer 108 comprising a hydrophobic open cell foam for directing air andair channel seals 110 that may comprise a hydrophobic closed cell foam.FIG. 5A shows an expanded schematic view and FIG. 5B shows an assembledschematic view of a exemplary module 150 comprising flexible panels 100and air channel layers 108. In FIGS. 5A-5B, there are four flexibleliquid desiccant heat and mass transfer panels that are separated by airchannel layers. Desiccant flow is shown through the desiccant flowdistributors 104 at one end, and air flow is shown perpendicular to thedesiccant flow. FIG. 6 shows an exemplary heat and mass transfer module150 comprising two end/support plates 152, plate connecting and gapadjustment features 154, one flexible LDAC panel 100 and two air flowchannel layers (not numbered). A header 156 may be formed from twolayers of film that are hot-melt sealed to both sides of the panel 110and hot-melt sealed together to form side seals. A tube may be placed inthe header to deliver fluid.

It is noted, in addition, that air and desiccant paths can also beconfigured in an in-line manner with the air and liquid desiccant inconcurrent or countercurrent flow. Automated or semi-automated processescan be used to make the panels and assembled modules in a very costeffective manner. The number of components in the panel and moduleassembly has been minimized by the embodiments herein.

It is expected that the flexible panels will be relatively insensitiveto the build-up of any dirt and debris on the surface of the poroushydrophilic separation layer. Air filters such as 3M Filtrete 2″Mini-pleat MERV 14 Commercial Air Filters will generally be used toprotect the panels. Any dirt and debris that impinges and collects onthe surface of the separation layer will wet out and may slowly increasethe effective thickness of the controlled thin film desiccant layer.However, diffusion of water molecules will still continue and it isexpected that only a small degradation in performance will occur overtime. It was demonstrated in the laboratory that panels could be rinsedclean of any dirt and debris if necessary.

The desiccant is biocidal and having the desiccant film located withinand on the surfaces of the hydrophilic membrane and at the air interfaceminimizes any chance of bio-fouling or bio-film build-up on the panel orin the module assembly.

The individual panels can be easily disassembled from the module andcompressed to “wring-out” desiccant for recovery and reuse. Panels canbe compacted in waste drums for transport and disposal. Panels andmodules can be incinerated.

Materials

Porous Hydrophilic Separation Layers

Hydrophilic separation layers may comprise a membrane, a woven mesh, ananofiber media, an electrospun fiber media, a glass fiber media, anonwoven melt blown fiber media, a corrosion-resistant metal, a ceramicmedia, or combinations thereof.

Many types of porous hydrophilic separation materials may be consideredfor use. Examples include blown melt fiber (BMF) materials made fromnylon or other polymers that have been pre or post treated to make thefiber surfaces hydrophilic. Extremely tightly woven mesh materials,nanospun and electrospun fiber media, and glass fiber media can also beconsidered. Sintered metal could be considered although corrosionresistance will need to be managed by material selection or by use ofcoatings due to the nature of the desiccant. Porous ceramic materialsare also candidates for the separation layer. Hydrophilicmicro-filtration and ultra-filtration membranes are very goodcandidates. Materials particularly well suited for the application canbe selected from the group of hydrophilic micro-filtration membranes.This group includes but is not limited to hydrophilic nylon (PA)membranes, hydrophilized polyethersulfone (PES) membranes, hydrophilizedpolysulfone (PS) membranes, hydrophilized polyvinylidene fluoride (PVDF)membranes, hydrophilic polyacrylonitrile (PAN) membranes hydrophilizedpolypropylene (PP) membranes, hydrophilized polyethylene (PE) membranes,hydrophilized polytetrafluorethylene (PTFE) membranes, hydrophilizedpolycarbonate (PC) membranes, and hydrophilized ethylenechlorotrifluoroethylene (ECTFE) membranes. Membranes can be naturallyhydrophilic, as is the case with PA membranes, or can be surfacemodified to render them hydrophilic. Many techniques forhydrophilization can be used including use of co-polymers and otheradditives in the polymer blend, coating with surfactants or otherhydrophilic materials, or grafting of hydrophilic groups to the membranesurfaces using free radical polymerization techniques or radiationgrafting.

It is important for the hydrophilic separation layer to have a high flowresistance as compared to the flow resistance of the desiccantdistributor. This creates preferential bulk flow down the desiccantdistributor located between the separation layers. It is preferable touse porous hydrophilic separation layers which have pore sizes of 10micron or less and with a flow resistance which minimizes the exteriorfilm coating which forms on the air channel side due to the very slighthydraulic pressure generated by the fluid column at the top of thepanel. It is more preferable to use porous hydrophilic separation layerswhich have pore sizes of 5 micron or less. Pore sizes can be measuredfor most materials using a capillary flow porometer. An exampleporometer is produced by Porous Materials, Incorporated of Ithaca, N.Y.

A particularly useful porous hydrophilic separation layer is amicro-filtration membrane made from nylon 6,6 as described in U.S. Pat.No. 6,513,666 entitled “Reinforced, Three Zone Microporous Membrane,”which is herein incorporated by reference. This type of membrane hasmultiple zones of different pore size and incorporates a scrim toprovide mechanical stability. In the particular embodiments prototypedto date, a membrane designated as BLA080 was used. This membrane has a1.2 micron zone on one side of the scrim, a 1.2 micron zone within thescrim, and a 0.8 micron zone on the other side of the scrim. Whenfabricated into the module, the orientation of the membrane wascontrolled so that the 0.8 micron zone is facing the air stream. Thesmaller pores have the highest capillary forces so it is advantageous tohave this zone controlling the thin film of desiccant at the desiccantair interface. The more open 1.2 micron zone facing the desiccant isbetter for insuring high mass diffusion of water into the bulk flow ofthe desiccant occurring in the desiccant distributor. Pore sizes andthicknesses of the zones of the membrane may be tailored for individualapplications to achieve desired mass and heat transfer across the thinliquid film in the membrane.

An exemplary membrane design would be to make a two zone membranecomprising a very open pore structure, say 3.0 micron, in the scrim fillzone, which is approximately 2.5 mils thick, and a tighter pore sizeadjacent zone, say 0.8 micron, which is 1 mil thick or less. The openpore zone would be positioned facing the desiccant side in a panel andthe tight pore zone would face the air stream. This design will minimizemass transfer resistance in the panel. A discussion of how to make anengineered micro-filtration membrane of this type is discussed in patentapplication WO 2013/154755 entitled “Thin Film Composite MembraneStructures,” hereby incorporated by reference.

In addition, unreinforced (no scrim) hydrophilic membranes would beuseful as well. An example of this type of membrane can be found in U.S.Pat. No. 6,706,184 entitled “Unsupported Multizone MicroporousMembrane,” which is incorporated herein by reference. In particular,polyethersulfone membranes produced with a thin tight pore size on theair channel side and a more open zone facing the desiccant distributorside would work well in the application.

Desiccant Flow Distributors

A desiccant flow distributor may include, but is not limited to ahydrophilic material that comprises a polymeric material, a naturalfiber, or combinations thereof. In some embodiments, the desiccant flowdistributor may comprise a hydrophilic polymer material that comprises amembrane, an open cell foam, a porous nonwoven material, and/or a porouswoven material. Some embodiments may use a hydrophilic polymer materialthat comprises a rail film, an extruded web material, an aperturedpolymeric film, or combinations thereof as the desiccant flowdistributor. The desiccant flow distributor may also comprise ahydrophilic natural fiber that comprises cellulose.

Open cell foams are typically made from polyurethanes. Both polyetherand polyester urethane foams are common. They can be hydrophilized byadding surfactants to the formulation or post treated to make themhydrophilic. The properties of the foam that are typically characterizedare density, compression deflection, compression set, pore sizeexpressed in pores per linear inch of material, tensile strength, tearstrength, air flow, and wet-out. Examples of useful hydrophilic opencell foams include Type HS or HydroZorb, from UFP Technologies,Georgetown, Mass.

Desiccant distributors can be rendered hydrophilic by various methodsknown in the art. For example: U.S. Pat. No. 6,548,727 (Foam/Filmcomposite medical articles, 2003); U.S. Pat. No. 5,254,301 (Process forpreparing a sheet of polymer-based foam, 1993); U.S. Pat. No. 4,957,810(Synthetic sponge-type articles having excellent water retention, 1990);and U.S. Pat. No. 3,781,231 (Physically reinforced hydrophilic foam andmethod of preparing same, 1973).

The shape of the hydrophilic open cell foam is also important incontrolling the uniform flow and distribution of the desiccant withinthe panel. An aspect of this invention is to add a series of features,for example, draw-and-drip features, at the bottom of the foam in thepanel. These features cause the desiccant to “draw” or “drip” uniformlyat the bottom of the panel and promote uniform flow throughout thepanel. Uniform flow is important in insuring efficiency of heat and masstransfer in the panel. It is advantageous for the desiccant to uniformlyabsorb heat and mass as it flows behind the porous hydrophilicseparation layer. Any type of channeling of the desiccant would causeuneven heat and mass absorption and reduced heat and mass exchangerefficiency. The features can be a series of crowns, points, or any othershape that promotes drawing or dripping of the desiccant at severalpoints at the bottom of the panel. If the panel is left straight acrossat the bottom, the effects of surface tension of the fluid interactingwith the bottom edge of the desiccant distributor may cause onedischarge stream to form, which may inhibit uniform desiccant flow.Example shapes are provided in FIGS. 7A-7D. FIG. 7A shows an exemplarydesiccant flow distributor 204 having rectangular shapes 205 atintervals to form draw-and-drip features extending from an end. FIG. 7Bshows an exemplary desiccant flow distributor 304 having square shapes305 at intervals to form draw-and-drip features extending from an end.FIG. 7C shows an exemplary desiccant flow distributor 404 havingtriangular shapes 405 at intervals to form draw-and-drip featuresextending from an end. FIG. 7D shows an exemplary desiccant flowdistributor 504 having semi-circular shapes 505 at intervals to formdraw-and-drip features extending from an end. Other materials can beconsidered for use as the desiccant distributor but these materials mustbe able to both wick and flow for them to properly function. Porouswicking materials that can also carry bulk flow include but are notlimited to cellulosic sponges, natural sponges, fabrics, and gauzes.

Air Channel or Turbulation Layers

It is advantageous to utilize a very open cell foam for the air channelbetween adjacent panels. A specifically useful type of very open cellfoam is a reticulated open cell foam, which has a substantially uniformpore size and a lattice that is wide open. These types of foams areuseful for providing a low pressure drop channel with controlledspacing. In addition, the shape of the foam adds some tortuosity andpromotes mixing of the air. The mixing of the air helps break up theboundary layer of the air with the desiccant thin film controlled in theporous hydrophilic separation layer. This promotes better heat and masstransfer between the air and the desiccant. It is also advantageous touse a hydrophobic material of construction in the air channel foam. Thisis helpful as it insures that any desiccant, condensation, or any othersource of liquid (water) that gets into the air channel preferentiallywets the porous hydrophilic separation layer and due to the thin filmcontrolled desiccant layer in and on the separation layer, is quicklyspread and absorbed into the desiccant flow stream. Essentially thewater has a zero contact angle with the desiccant and it is impossiblefor a drop to form on the separation layer surface. In addition, thethickness, pore size, and tortuosity of the air channel layer can beoptimized to balance air flow pressure drop with heat and mass transferperformance. Excessive pressure drops in the air channel can lead toparasitic losses due to the fan energy consumed in a LDAC system. Thepanel space can be determined with analytical and computational modelingto optimize the panel spacing which is related to the power density ofan assembled module.

An example of an effective material for use as air channel spacers ispolyester filter foam S-10 from New England Foam Products, LLC,Hartford, Conn.

It is not required to have a porous air channel spacer in an assembledmodule. The panels can instead be affixed at set intervals or nonporousspacers can be used at each end of the panel to control the width of theair gaps between panels. In this manner, air gaps will be provided in anassembled module.

An exemplary air channel layer may comprise a polypropylene rail filmwith adhesive (structured film) as disclosed in U.S. Pat. No. 6,986,428to common assignee 3M Innovative Properties Company and herebyincorporated by reference. This film may be useful to make an air sideseparator and can be designed to provide air channels with low pressuredrop and also face support for the flexible LDAC membrane panel whenassembled into a module. This film can also be made without theadhesive. In other words, the full geometry of the film can be made ofone material such as polypropylene or polyethylene. It is also possibleto make the air channel layers out of many other plastics or metals inthe form of plates. The air channel layers or plates can be flexible orrigid. The plates can be machined, thermoformed, extruded, cast, orproduced in a number of other ways. Rail type films may be modified withsurface features to produce mixing of any fluid (i.e. air, liquiddesiccant) which flows down the channels of the film. An exemplary layermay comprise a polymer film comprising micromixing surface features suchas those disclosed in commonly-assigned U.S. Ser. No. 61/736,729 filedDec. 13, 2012, entitled “Constructions for Fluid Membrane SeparationDevices” and incorporated herein by reference.

Other materials such as nettings and apertured films can also beconsidered for use as air channel spacers.

Desiccant Distribution Header

The header should promote uniform flow at the top of the panel and it isadvantageous to feed the desiccant through a slot or a series of holes.This will insure even distribution at the top of the panel. This worksin coordination with the drawing and dripping features at the bottom ofthe panel to insure uniform desiccant flow behind the porous hydrophilicseparation layer. An exemplary header may be made from nonporous polymerfilm. For example, two layers of film may be hot-melt sealed to bothsides of a panel and hot-melt sealed to form side seals. A tube may beplaced in the header to deliver fluid.

Before describing several exemplary embodiments of the invention, it isto be understood that the invention is not limited to the details ofconstruction or process steps set forth in the following description.The invention is capable of other embodiments and of being practiced orbeing carried out in various ways.

EXAMPLES Example 1

A double-sided porous, flexible heat and mass transfer panel incombination with an air channel layer is made by assembling:

an air channel layer of a reticulated polyester urethane foam at 10pores per inch (PPI), a density of 1.9 lbs/cu ft, 25% CFD is 0.45 psi,16 psi tensile strength, elongation 170%, tear strength 4.5 lbs/in,compression set at 50% deflection is 15, volumetric flow rate is 23,0.25″ thick—product S-10 from New England Foam;

two porous separation layers of an engineered membrane comprising: ahydrophilic nylon 6,6 membrane, a multi-zone structure with a 1.2 micronzone on the desiccant side and a 0.80 micron zone on the air side,membrane is reinforced with a nonwoven scrim in a center zone—from 3MPurification, U.S. Pat. No. 6,513,666;

a desiccant flow distributor of a polyether urethane foam, double cell,hydrophilized with a surfactant, 0.25″ thick—Type HS from UFPTechnologies, having draw-and-drip features in a series of shapes oftriangular point geometry at the bottom of the panel;

tape side seams comprising multiple layers of a closed cell acrylic foambacked adhesive tape; and

a desiccant distribution header comprising a multi-layer nonporouspolymer film, using a general purpose hot melt to adhere to the membraneand to form side seals of the header.

Example 2

A single-sided porous, flexible heat and mass transfer panel incombination with an air channel layer was made by assembling:

an air channel layer of a reticulated polyester urethane foam at 10pores per inch (PPI), a density of 1.9 lbs/cu ft, 25% CFD is 0.45 psi,16 psi tensile strength, elongation 170%, tear strength 4.5 lbs/in,compression set at 50% deflection is 15, volumetric flow rate is 23,0.25″ thick—product S-10 from New England Foam;

one porous separation layer of an engineered membrane comprising: ahydrophilic nylon 6,6 membrane, a multi-zone structure with a 1.2 micronzone on the desiccant side and a 0.80 micron zone on the air side,membrane is reinforced with a nonwoven scrim in a center zone—from 3MPurification, U.S. Pat. No. 6,513,666;

one non-porous separation layer comprising a polyethylene film;

a desiccant flow distributor of a polyether urethane foam, double cell,hydrophilized with a surfactant, 0.25″ thick—Type HS from UFPTechnologies, having draw-and-drip features in a series of shapes oftriangular point geometry at the bottom of the panel;

tape side seams comprising multiple layers of a closed cell acrylic foambacked adhesive tape; and

a desiccant distribution header comprising a multi-layer nonporouspolymer film, using a general purpose hot melt used to adhere to themembrane and to form the side seals of the header.

Example 3

A double-sided porous, flexible heat and mass transfer panel was made byassembling:

two porous separation layers of an engineered membrane comprising ahydrophilic nylon 6,6 membrane, a multi-zone structure with a 1.2 micronzone on the desiccant side and a 0.80 micron zone on the air side,membrane is reinforced with a nonwoven scrim in a center zone—from 3MPurification, U.S. Pat. No. 6,513,666;

a desiccant flow distributor of a polyester urethane foam, hydrophilizedwith a surfactant, 0.25″ thick—type HydroZorb from UFP Technologies,having draw-and-drip features in a series of shapes of triangular pointgeometry at the bottom of the panel;

ultrasonically-welded side seams using a Branson Ultrasonic Welder withthe following settings:

Weld Pressure—50 psi

Weld Time—0.80 sec

Weld Hold Time—0.5 sec

Trigger Force—set at 12

Down speed—set at 30

Amplitude—set at 100%; and

desiccant distribution header where an extra membrane was allowed forabove the top of the ultrasonically welded side seals enabling theformation of a pocket that served as a desiccant header. A tube ormultiple tubes may be inserted into this pocket and desiccant may bepumped directly into the panel.

Example 4 Testing

Flow Test 1

A panel as described in Example 2 and with an active membrane area of 6″wide×7″ high×one side was tested with water to determine the flowcapability of the panel. Active membrane area is defined as the wettedmembrane surfaces which have fluid flowing behind it on the desiccantdistributor side and does not include any side seams, header area, orthe draw-and-drip features at the bottom of the panel. A peristalticpump was used to deliver the water to the top of the panel and the tubewas situated to allow the liquid to drop into the header at the centerof the panel. The maximum flow rate of the pump was 88.5 ml/min and thepanel could handle this flow rate without issue. Liquid did not build upin the header but rather was drawn into the panel and spread by thehydrophilic open cell foam desiccant distributor. The triangularfeatures started to drip as the liquid began reaching the bottom of thepanel. Within two minutes of starting the flow, the dripping wasuniformly distributed across the triangular points indicating uniformflow.

Flow Test 2

A panel as described in Example 3 and with an active membrane area of 6″wide×7″ high×two sides was tested with water to determine the flowcapability of the panel. A peristaltic pump was used to deliver thewater to the top of the panel and the tube was situated to allow theliquid to drop into the header at the center of the panel. The maximumflow rate of the pump was 88.5 ml/min and the panel could handle thisflow rate without issue. Liquid did not build up in the header butrather was drawn into the panel and spread by the hydrophilic open cellfoam desiccant distributor. The triangular draw-and-drip featuresstarted to drip as the liquid began reaching the bottom of the panel.Within two minutes of starting the flow, the dripping was uniformlydistributed across the triangular points indicating uniform flow.

Flow Uniformity Test

The flexible panel tested in Flow Test 1 was tested with a solution ofmethylene blue dye in water to observe the flow distribution within thepanel. The panel was mounted without an air channel spacer on one sideand with the nonporous film against the plexiglass end plate on themodule holder so the flow patterns could be observed. A sequence ofphotos was taken in increments over the first two minutes of liquid flowand uniform spreading was observed. This was done with a single pointfeed at the top of the panel. It is thought that a header with multiplefeed points or use of a slot feeder would improve the uniformity at thetop corners of the panel to insure uniform flow through the entire faceof the active membrane area.

Freeze/Thaw Test

The panel described in Example 2 and flow tested in Flow Test 1 and inthe Flow Uniformity Test was subject to two cycles of freeze/thawtesting. In the first cycle, the panel was completely saturated withwater and frozen solid to 10° F. in a freezer compartment over a periodof approximately 5 hours. It was frozen as a panel and not restrained ina holder. It was then removed from the freezer and while still frozen,rapidly immersed in a pan of hot water at approximately 125° F. Afterstabilizing at the bath temperature, the panel was removed and visuallyexamined for damage. No damage was observed. The panel then wasremounted into a holder with a foam air channel spacer on the membraneside and the non-porous film side facing the plexiglass end plate on theholder. The panel was tested as described in Flow Test 1 and performednormally.

This same panel was then mounted in a holder with air channel spacers oneach side. It was fully saturated with water and then put back in thefreezer. It was frozen solid overnight to 10° F. (approximately 16hours), removed and then rapidly immersed into 125° F. water as before.Again, no visual damage was observed when visually inspected and thepanel was retested for flow performance. It tested normally.

This experiment indicates that the flexible panel design as described inthis invention can withstand a high level of thermal and mechanicalstress without damage or reduction in functional performance.

Condensation Control and Desiccant Retention

The panel as described in Example 2 was assembled into a holder. Waterwas introduced into the header as described in Flow Test 1. A pipettewas then used to introduce water droplets as a surrogate to condensationformation on the membrane surface. This simulated the case when colddesiccant is introduced into the top of the panel while warm humid airis delivered down the air channel. When the surface temperature of thethin film desiccant controlled in the porous hydrophilic separationlayer is below the dew point of the passing air, condensation will format the interface. The surrogate water droplets demonstrated that whenthey touched the active membrane surface, they were immediately spreadacross the face of the panel. Droplets were unable to form on thesurface. This demonstrates that any condensation formation at theliquid/air interface will be immediately spread and absorbed into thethin film layer controlled in and on the panel. In this design, there isalso no chance for capillary condensation as the separation layeroperates in a totally wetted form.

In addition, a pipette was used to introduce water droplets into themiddle of the hydrophobic foam used to create the air channel. Dropswere able to “hang up” in the middle of the channel without touching amembrane surface. It is unlikely that condensation would form in themiddle of the air channel, but if this happened, it was noted that whenair was blown down the air channel, the drops would follow the latticeof the foam for a short distance until the water hit the surface of themembrane. Since the membrane is essentially a liquid film, and thelattice is hydrophobic, the preferential wetting caused the waterdroplets to be immediately pulled off of the lattice and onto the panel.The panel described in this invention will be tolerant of condensationformation based on the surface tension, capillary action andhydrodynamic control of a falling column of liquid desiccant created bythe integrated panel design. This testing also demonstrates that anydesiccant is firmly sequestered in the panel and will not be aerosolizedby the flow of air expected in the application of the panels in an LDACsystem.

Example 5

A double-sided porous, flexible heat and mass transfer panel was made byassembling:

two porous separation layers of an engineered membrane comprising ahydrophilic nylon 6,6 membrane, a multi-zone structure with a 1.2 micronzone on the desiccant side and a 0.80 micron zone on the air side,membrane is reinforced with a nonwoven scrim in a center zone—from 3MPurification, U.S. Pat. No. 6,513,666;

a desiccant flow distributor—0.025″ thick nylon Naltex asymmetricdiamond netting;

side seams comprising double sided pressure sensitive adhesive (PSA)tape;

a desiccant distribution header was created by having extra membrane atthe top to perform a pocket. A tube was inserted at the center of thepocket during flow testing.

Example 6

A double-sided porous, flexible heat and mass transfer panel was made byassembling:

two porous separation layers (2) of an engineered membrane comprising ahydrophilic nylon 6,6 membrane, a multi-zone structure with a 1.2 micronzone on the desiccant side and a 0.80 micron zone on the air side,membrane is reinforced with a nonwoven scrim in a center zone—from 3MPurification, U.S. Pat. No. 6,513,666;

a desiccant flow distributor—0.010″ thick polypropylene Naltexasymmetric diamond netting;

side seams comprising double sided pressure sensitive adhesive (PSA)tape;

a desiccant distribution header was created by having extra membrane atthe top to perform a pocket. A tube was inserted at the center of thepocket during flow testing.

Example 7 Testing

Flow testing of the panels of Examples 5-6 with methylene blue dyesolution showed liquid distribution patterns behind the hydrophilicnylon membrane that were less uniform than those patterns shown forExamples 2-3. Lower fluid flow carrying capacities compared to Examples2-3 utilizing the hydrophilic open cell foam were noted. It is thoughtthat improved capillary action and liquid spreading could be achieved bythe use of diamond netting or even apertured films having thinnerapertures and channels and more hydrophilic materials relative to whatwas tested in Examples 5-6. It is expected, however, that such thinnerand more hydrophilic materials may result in lower fluid carryingcapacities per unit area of active porous hydrophilic separation layer.

In contrast, the hydrophilic open cell foams provide a good balance ofproperties which allow for superior flow distribution while stillmaintaining high bulk flow of the liquid. When applying this paneldesign for LDAC applications, it will be extremely beneficial toaccurately control the volume of bulk fluid while maintaining a stablethin film in and on the porous hydrophilic separation layer. The bulkfluid flow will provide ample heat transfer capacity between theconditioner and the regenerator in a LDAC system while accuratelycontrolling the thin film desiccant layer within and on the surfaces ofthe porous hydrophilic separation layer.

Reference throughout this specification to “one embodiment,” “certainembodiments,” “one or more embodiments” or “an embodiment” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe invention. Thus, the appearances of the phrases such as “in one ormore embodiments,” “in certain embodiments,” “in one embodiment” or “inan embodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It will be apparent to those skilled in the art thatvarious modifications and variations can be made to the method andapparatus of the present invention without departing from the spirit andscope of the invention. Thus, it is intended that the present inventioninclude modifications and variations that are within the scope of theappended claims and their equivalents.

1. A heat and mass transfer panel for water vapor exchange with a liquiddesiccant, the panel comprising: a desiccant flow channel defined by afirst flexible layer and a second flexible layer, at least one of whichcomprises a flexible hydrophilic water vapor-permeable separation layer;a desiccant inlet and a desiccant outlet to the desiccant flow channel;and a flexible desiccant flow distributor located in the desiccant flowchannel.
 2. The heat and mass transfer panel of claim 1, wherein boththe first and the second flexible layers comprise a flexible hydrophilicwater-vapor permeable separation layer.
 3. The heat and mass transferpanel of claim 1, wherein the first flexible layer comprises a flexiblehydrophilic water-vapor permeable separation layer and the secondflexible layer is a non-porous layer or a hydrophobic water-vaporpermeable separation layer.
 4. The heat and mass transfer panel of claim1, wherein the flexible hydrophilic water-vapor permeable separationlayer or layers independently comprise a membrane, a woven mesh, ananofiber media, an electrospun fiber media, a glass fiber media, anonwoven melt blown fiber media, a corrosion-resistant metal, a ceramicmedia, or combinations thereof.
 5. The heat and mass transfer panel ofclaim 1, wherein the flexible hydrophilic water-vapor permeableseparation layer or layers independently comprise a micro-filtration oran ultra-filtration membrane comprising a hydrophilic nylon (PA)membrane, a hydrophilized polyethersulfone (PES) membrane, ahydrophilized polysulfone (PS) membrane, a hydrophilized polyvinylidenefluoride (PVDF) membrane, a hydrophilic polyacrylonitrile (PAN)membrane, a hydrophilized polypropylene (PP) membrane, a hydrophilizedpolyethylene (PE) membrane, a hydrophilized polytetrafluorethylene(PTFE) membrane, a hydrophilized polycarbonate (PC) membrane, ahydrophilized ethylene chlorotrifluoroethylene (ECTFE) membrane, orcombinations thereof.
 6. The heat and mass transfer panel of claim 1,wherein the desiccant flow distributor comprises a hydrophilic materialthat comprises a polymeric material, a natural fiber, or combinationsthereof.
 7. The heat and mass transfer panel of claim 6, wherein thedesiccant flow distributor comprises a hydrophilic polymer material thatcomprises a membrane, an open cell foam, a porous nonwoven material, aporous woven material, or combinations thereof.
 8. The heat and masstransfer panel of claim 1, wherein the desiccant flow distributorcomprises one or more draw-and-drip features at an outlet end of thedistributor; wherein the draw-and-drip features are effective forfacilitating uniform flow through the panel.
 9. The heat and masstransfer panel of claim 1 further comprising an air channel layer. 10.The heat and mass transfer panel of claim 1 further comprising adesiccant distribution header.
 11. A heat and mass transfer modulecomprising: one or more panels of claim 1 assembled among one or moreair channel layers or air gaps; and an air inlet and an air outlet. 12.The heat and mass transfer module of claim 11 further comprising two endplates between which the one or more panels and the one or more airchannel layers are assembled.
 13. A method for water vapor exchangebetween air and a liquid desiccant, the method comprising: contactingthe panel of claim 1 with air having a water vapor pressure differentfrom the equilibrium vapor pressure in a desiccant flowing through thedesiccant flow channel; wherein the humidity of the air after contactwith the panel is different from the humidity before contact with thepanel.
 14. A desiccant flow distributor comprising: a hydrophilicstructure comprising a polymeric material, a natural fiber, orcombinations thereof; and one or more draw-and-drip features at anoutlet end of the structure; wherein the draw-and-drip features areeffective for facilitating uniform flow therethrough.
 15. The desiccantflow distributor of claim 14 comprising a hydrophilic polymeric materialthat comprises a membrane, an open cell foam, a porous nonwovenmaterial, a porous woven material, or combinations thereof.
 16. Thedesiccant flow distributor of claim 14, wherein the one or moredraw-and-drip features comprise a series of edges such that the linearedge near or at the outlet end of the structure is longer than thelinear edge at the inlet end of the structure.
 17. The desiccant flowdistributor of claim 14, wherein the one or more draw-and-drip featurescomprise a series of shapes defined by or at the edges of the outletend.